Button image does not appear in UWP app

Hi,
My app's XAML:
<?xml version="1.0" encoding="utf-8" ?>

  <Button Image="clock.png"/>

</ContentPage>

The code above isn't correct, because when I paste it into the forum, it get's cut. That's why you don't see ContentPage opening - it's there in my app. The importnat thing to know is that I have just one button with an image - and that's it.

It's very simple xaml and it works well on Android. Unfortunately it doesn't work in UWP.
In case of Android I put clock.png into Resources/drawable.
In case of UWP I put clock.png into project's root folder, just like described here:https://developer.xamarin.com/guides/xamarin-forms/working-with/images/

Unfortunately, UWP app has a button without any content. What's wrong?